1. Field of the Invention
This invention relates to an improved molding process and to hollow plastic articles made thereby.
2. Background
Molding is a process in which melted or plasticized polymeric material is forced into a mold where it is held until removed in a solid state having a shape which duplicates the cavity of the mold. The process conventionally consists of three basic operations: (1) raising the temperature of the plastic to a point where it will flow under pressure through a gate and into the mold cavity, (2) allowing the plastic to solidify in the mold (conventionally referred to as hold time), and (3) opening the mold to eject the plastic frozen in the shape of the mold cavity.
During molding, the macromolecules of most polymer melts orient themselves in the direction of the flow, conventionally referred to as the machine direction, MD, as a result of shear stress. Orientation in the MD is of course experienced to a greater degree by molecules close to the mold wall than by molecules more distant from the wall because polymer molecules in close proximity to the wall are subject to greater shear stress. This situation leads to a skin-core structure in which the orientation of the molecules of the core layer is essentially random because of the lower shear stress experienced by molecules more distant from the mold walls. Further, the skin-core morphology is augmented because molecules in the skin layer come into contact with the cold mold and freeze sooner compared to the molecules in the core layer which have a longer time to relax.
While the mechanical performance of a molded polymeric article is dominated by its chemical nature, the effect of morphological structure has been investigated, and better mechanical properties in the skin layer than in the core layer have been attributed to molecular orientation. Since many injection molded articles, particularly medical articles, have thin walls, they are subject to stress cracking on impact. Accordingly, there is a need in the molding art for a process to further enhance mechanical properties. This invention addresses this need.